Somatostatin (SRIF) is a neuropeptide which has been proposed as the neurotransmitter or neuromodulator substance although the only known function of this peptide is as a neurohormone. This peptide, along with several others, has been demonstrated by previous investigators in the retinas of all species examined. Our preliminary studies demonstrate the presence of this peptide in neuronal perikarya and fiber plexuses in the superior colliculus, pretectum, lateralis posterior, lateral geniculate nucleus and visual cortex, and suggest that in the visual system SRIF neurons may constitute both a projection neuron as well as interneuron population. Since electrophoretic applications of SRIF have been shown to both inhibit and excite different populations of neurons, and SRIF is known to effect behavior, it is proposed to investigate the distribution and organization of SRIF neurons, and the role of SRIF in the visual system. We will employ light microscopic immunocytochemistry for the demonstration of SRIF in neurons and fibers in the visual system of the adult and during development. To determine the efferent and afferent connections of these neurons we will utilize the placement of discrete lesions, anterograde transport of 3H-amino acids and retrograde transport of HRP and/or fluorescent dyes, in combination with SRIF immunocytochemistry. The relationship of SRIF neurons to other transmitters will be studied by combining immunocytochemistry for SRIF with immunocytochemistry for the localization of other transmitters and biogenic amines, and autoradiography to demonstrate GABAergic neurons. The proposed investigation will use the rat and the cat as experimental animals and is designed to generate data which will provide a basis for evaluating the role of SRIF in the visual system.